Babies born prematurely take fewer risks when they grow up, a study has found.

US researchers followed very low birth-weight babies up to the age of 20 and found they were much less likely to use alcohol or illegal drugs, have sex or become pregnant than those born at normal weight.

Experts suggest this may be because parents or the children themselves restrict their behaviour because of their early health problems.

The study is said to be the largest and most comprehensive follow-up to date of very low birth weight infants, since their survival was made possible by the clinical advances in newborn care in the late 1970s.

We believe that this limited risk-taking behaviour may result from increased parental monitoring

Professor Maureen Hack, Case Western Reserve University School of Medicine

Doctors from the Rainbow Babies & Children's Hospital of University Hospitals of Cleveland and Case Western Reserve University School of Medicine looked at how 242 babies, born between 1977 and 1999, fared.

The babies, born between 1977 and 1999, weighed on average 1,179 grams (just under 2.5 lbs), and were born on average during the 29th week of pregnancy.

A pregnancy is considered full term at 37 weeks.

The children's academic and cognitive achievements, rates of serious illness and what risky behaviour they indulged in.

Educational difficulties

Those who were a very low birth-weight were more likely to have one or more chronic health problems, especially neurosensory conditions such as cerebral palsy, blindness, or deafness, and shorter height than normal weight babies.

Very low birth-weight babies also had lower IQs (87% versus 92% for normal-weight babies) and had lower scores on academic achievement tests at age 20, according to the study published in the New England Journal of Medicine.

They were also less likely to have graduated from high school - 74% compared with 83%, or to have enrolled in a four-year college course according to the study, which was partly funded by the National Institute of Child Health and Development (NICHD).

However, 51% have IQ scores within the normal range; 74% have completed high school and 41% are pursuing post -secondary education.

Researchers believe the study's findings can also be applied to babies of similar weights born today because rates of neurodevelopmental complications have not changed substantially since the 1970s.

Maureen Hack, professor of paediatrics at Case Western Reserve University School of Medicine, who led the study, said: "We believe that this limited risk-taking behavior may result from increased parental monitoring of very-low-birth-weight."

Unexpected achievements

In an accompanying editorial in the New England Journal, Dr Marie McCormick, Harvard School of Public Health and Dr Douglas Richardson, of the Beth Israel Deaconess Medical Center said even though premature babies were more likely to grow up to lag behind their peers, it was encouraging to see a large number of them did do well.

"What is unexpected is their relative success despite such difficulties.

"These are children who face substantial challenges, beginning very early in their schooling, because of cognitive and behavioral problems.

"The fact that they are almost as successful as the members of the normal-birth-weight comparison group in completing school and at least as successful in avoiding risk-taking behavior speaks to a resilience in these young persons and their families that needs to be examined further," they added.

Dr Duane Alexander, director of the NICHD said: "It's very probable that their avoidance of risky behaviors is due to more attentive parenting.

"Other NICHD research has shown that children whose parents are actively involved in their lives are less likely to engage in risky behaviors than children whose parents are less involved."

Robin Murray, professor of psychiatry at the Institute of Psychiatry in London, who has also carried out research into how premature babies fare as they grow older, told BBC News Online: "It might be that parents protect them, but it might also be that these children know themselves that they have had a difficult time and restrict their own behavior".

He said premature birth was a "tremendously important" public health problem, because ever-smaller babies were being helped to survive and the consequences for their health had to be investigated.